Value documents are understood here to be sheet-shaped objects that are to represent for example a monetary value or an authorization and are hence not to be producible at will by unauthorized persons. Hence, they have features that are not easily produced, in particular copied, whose presence is an indication of authenticity, i.e. production by an authorized body. Important examples of such value documents are chip cards, coupons, vouchers, checks and in particular bank notes.
Such value documents, in particular for example bank notes, are often processed by machine. Firstly, upon the transport of such value documents along a transport path of a value-document processing apparatus it is often necessary to check whether, or when, a value document passes a predetermined place. For such a check or detection of a value document there can be employed for example optical sensors in the form of light barriers which have a radiation source for optical radiation of at least one predetermined wavelength as a transmitter and a photodetector for radiation of the predetermined wavelength as a receiver. To avoid soiling of these components for example by dust, or damage by value documents being transported past, there is often located between said components and the transport path a sensor window which is transparent to the employed optical radiation. However, during operation of the apparatus said sensor window also becomes soiled after a certain operation period and must then be cleaned to guarantee trouble-free operation.
Corresponding problems occur with optical sensors serving to detect at least one optical property of value documents, in particular to recognize the type of value documents, for example the denomination of bank notes, or the authenticity of value documents. These also have, for example between the transport path and a detector, at least one sensor window transparent to the employed optical radiation which protects at least part of the sensor, for example the detector, but can itself become soiled and then impair the functioning of the sensor.
For keeping clean the sensor windows they can be periodically cleaned, but this can be elaborate.